The compound 4,4' difluorobiphenyl has been long used as an insecticide and more recently, has found use as a monomer in the preparation of high performance polymers. The present invention is a new and improved method for this compound.
In the past, 4,4'-difluorodiphenyl has been prepared from 4,4'-biphenyl-bis-diazonium-piperidide (by diazotizing benzidine and coupling with piperidine) and concentrated hydrofluoric acid; by the action of sodium on para-fluorobromobenzene in either; from benzidine by tetrazotization and decomposing the biphenyl-bis-diazonium salt with concentrated hydrofluoric acid (and even ferric chloride has been present to stimulate this reaction); and by the prolonged contact of the vapors of fluorobenzene with a red hot wire. However, prior to the present invention, the most satisfactory method for making 4,4'-difluorobiphenyl was by far the method described by G. Schiemann and W. Winkelmuller, published in "Organic Syntheses", Collected Volume 2, Page 188". The reaction therein proceeds as depicted below: ##STR1##
Commercial benzidine and concentrated hydrochloric acid are reacted to form the dihydrochloride. The benzidine dihydrochloride is tetrazotized and concurrently a mixture of boric acid and hydrofluoric acid is slowly added and stirred with a lead rod. A thick paste of 4,4'-biphenylene-bis-diazonium borofluoride forms. The product is collected and washed consecutively with alcohol and ether and dried. The product is decomposed with heat and the black residue is steam distilled to obtain a pure white compound to obtain the 4,4'-difluorobiphenyl in yields of 80 to 81% based upon the tetrazonium borofluoride, or 54 to 56% based upon the benzidine used. The raw materials for this reaction are inexpensive and the yields are commercially acceptable. However, the initial benzidine has become so highly regulated that it is practically unavailable for commercial use.
A facsimile method for synthesizing 4,4'-difluorobiphenyl is described by McKillop & Elsom, published in "Tetrahedron, Volume 26, Page 4041 (1970)". They describe the production of this compound in 77% yield from the magnesium Grignard of 4-bromofluorobenzene with thallium (I) bromide. Unfortunately, large quantities of toxic thallium compounds are produced as by-products and accordingly, this method is commercially unacceptable.
Accordingly, a relatively non-toxic reaction, from commercially available ingredients having either a low raw material costs and/or substantial commercial yields in the production of 4,4'-difluorobiphenyls would be a substantial advancement in the art.